No lead package with heat spreader

ABSTRACT

A no-lead electronic package including a heat spreader and method of manufacturing the same. This method includes the steps of selecting a matrix or mapped no-lead lead frame with die receiving area and leads for interconnect; positioning an integrated circuit device within the central aperture and electrically interconnecting the integrated circuit device to the leads; positioning a heat spreader in non-contact proximity to the integrated circuit device such that the integrated circuit device is disposed between the leads and the heat spreader; and encapsulating the integrated device and at least a portion of the heat spreader and leads in a molding resin.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No.11/670,650, filed Feb. 2, 2007, which claims the benefit of U.S.Provisional Application No. 60/777,316, filed Feb. 28, 2006, thedisclosures of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to packages for encapsulating one or moresemiconductor devices, and more particularly to a method for theassembly on a no-lead package having exceptional thermal performance.

(2) Description of the Related Art

In lead frame based semiconductor packages, electrical signals aretransmitted between at least one semiconductor device (die) and externalcircuitry, such as a printed circuit board, by an electricallyconductive lead frame. The lead frame includes a number of leads, eachhaving an inner lead end and an opposing outer lead end. Inner lead endsare electrically interconnected to input/output (I/O) pads on the dieand outer lead ends provide terminals outside the package body forinterconnection to external circuitry. When the outer lead endterminates at the face of the package body, the package is known as a“no lead” package. If the outer leads extend beyond the package bodyperimeter, the package is referred to as “leaded.” Examples of wellknown no-lead packages include quad flat no lead (QFN) packages whichhave four sets of leads disposed around the perimeter of the bottom of asquare package body and dual flat no lead (DFN) packages which have twosets of lead disposed along opposite sides of the bottom of a packagebody. Interconnection of the die to the inner lead ends is typicallyperformed using wire bonding, tape automated bonding (TAB) or flip chipbonding. In wire bonding or TAB bonding, the inner lead ends terminate adistance from the die and are electrically interconnected to I/O pads onan electrically active face of the die by small diameter wires orconductive tape. The die may be supported by a die pad which issurrounded by the leads. In flip chip bonding, the inner lead ends ofthe lead frame extend beneath the die and the die is flipped so that theI/O pads on the electrically active face of the die contact the innerlead ends by a direct electrical contact, such as a solder joint.

A representative QFN package and its method of manufacture is more fullydisclosed in commonly owned U.S. patent application Ser. No. 10/563,712published as PCT International Application No. WO2005/017968 A2 on Feb.24, 2005. The disclosure of U.S. patent application Ser. No. 10/563,712is incorporated by reference in its entirety herein.

An ongoing objective for the designers of no lead semiconductor packagesis better thermal management. That is, the ability to remove heat fromthe electrically active semiconductor die. The QFN is one of the bestlead frame based packages in terms of thermal management and cost, butas integrated circuit devices become more complex, there is a need forimproved thermal and electrical performance. Among the options availablein the market are the use of heavy wires and metal ribbons to conductheat away from the integrated circuit die.

The use of a heat spreader in a leaded package is disclosed in U.S. Pat.No. 5,608,267 to Mahulikar et al. The use of the heat spreader with asubstrate based package is disclosed in U.S. Pat. No. 5,977,626 to Wanget al. and U.S. Pat. No. 6,432,749 to Libres. The disclosures of U.S.Pat. Nos. 5,608,267; 5,977,626 and 6,432,749 are all incorporated byreference in their entireties herein.

None of the prior art designs include a no-external lead, lead framebased package having a heat spreader. Such a package would have enhancedthermal performance as compared to the QFN and other no-lead typepackages presently known.

BRIEF SUMMARY OF THE INVENTION

One aspect of the invention is a method for the manufacture of a no-leadelectronic package. The method includes the following: providing a leadframe having desired features including a plurality of leads terminatingabout a central aperture; positioning an integrated circuit devicewithin the central aperture and electrically interconnecting theintegrated circuit device to the leads; positioning a heat spreader innon-contact proximity to the integrated circuit device such that theintegrated circuit device is disposed between the leads and the heatspreader; and encapsulating the semiconductor device and at least aportion of the heat spreader and leads in a molding resin.

Another aspect of the invention is a semiconductor package, whichincludes the following: a plurality of leads having inner ends and outerends disposed about a centrally disposed die pad with a plurality of diepad tie bars extending outward therefrom; an integrated circuit devicehaving an electrically inactive face bonded to the die pad andelectrically active face electrically interconnected to the inner leadsby wires or TAB bonds; a heat spreader in non-contact proximity to theelectrically active face whereby the integrated circuit device isdisposed between the die pad and the heat spreader; and a molding resinencapsulating the integrated circuit device, at least a portion of theheat spreader and all but a planar surface of the die pad and the outerends.

Yet another aspect of the invention is a semiconductor package, whichincludes the following: a plurality of leads having inner ends and outerends disposed about a centrally disposed aperture; an integrated circuitdevice spanning the aperture and having an electrically active facedirectly bonded to the inner ends of the plurality of leads by a solder;a heat spreader in non-contact proximity to an electrically inactiveface of aid integrated circuit device whereby the integrated circuitdevice is disposed between the plurality of leads and the heat spreader;and a molding resin encapsulating the integrated circuit device, atleast a portion of the heat spreader, and all but a planar surface ofthe die pad and the outer ends.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, the drawings show a formof the invention that is presently preferred. However, it should beunderstood that the present invention is not limited to the precisearrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 is a top planar view of a lead frame matrix as known from theprior art for use in the invention;

FIG. 2 is a top planar view of a heat spreader matrix for use in thelead frame array and semiconductor die assembly;

FIG. 3 is a cross-sectional view of the heat spreader array of FIG. 2;

FIG. 4 is a cross-sectional representation illustrating the heatspreader array bonded to the lead frame array and semiconductor dieassembly;

FIG. 5 is a cross-sectional representation of a molded package arrayformed by the process of the invention;

FIG. 6 is a cross-sectional representation of singulated wire-bondedpackages formed by the process of the invention;

FIG. 7 illustrates in cross-sectional representation a process sequencefor a wire-bonded package in accordance with the invention;

FIG. 8 illustrates in cross-sectional representation a package formedaccording to the method of the invention;

FIG. 9 illustrates in cross-sectional representation a package formedaccording to the method of the invention;

FIG. 10 illustrates in cross-sectional representation a package formedaccording to the method of the invention;

FIG. 11 illustrates in cross-sectional representation a package formedaccording to the method of the invention;

FIG. 12 illustrates in cross-sectional representation a package formedaccording to the method of the invention;

FIG. 13 illustrates a method to assemble a flip-chip bonded packageformed by the process of the invention;

FIG. 14 is a cross-sectional representation of a flip-chip bondedpackage formed by the process of the invention;

FIG. 15 is a cross-sectional representation of another flip-chip bondedpackage formed by the process of the invention;

FIG. 16 is a cross-sectional representation of another flip-chip bondedpackage formed by the process of the invention;

FIG. 17 is a cross-sectional representation of another flip-chip bondedpackage formed by the process of the invention;

FIG. 18 is a cross-sectional representation of another flip-chip bondedpackage formed by the process of the invention;

FIG. 19 is a cross-sectional representation of a heat spreader flangeshowing an alignment feature;

FIG. 20 is a cross-sectional representation of a heat spreader flangeshowing an alternative alignment feature; and

FIG. 21 is a cross-sectional representation of a lead showing analignment feature.

DETAILED DESCRIPTION

FIG. 1 illustrates in top planer view a matrix 10 of lead frames asknown from the prior art. Typically, the lead frames are formed from anelectrically conductive material that is amenable to controlled chemicaletching. Suitable materials include copper and copper alloys,iron-nickel alloys, and aluminum and aluminum alloys. Package featuresdefined by the etch include die pads 14, leads 16 and tie bars 18. It isnoted that not all features are required for every type package. Forexample, the die pad 14 is optional in a flip chip package. The matrixforms a repetitive array of package features such that on completion ofthe assembly process, the array is singulated to a plurality ofindividual packages.

A heat spreader that improves the thermal capability of the QFN packageis illustrated in top planer view in FIG. 2 and cross-sectionalrepresentation in FIG. 3. A metallic sheet is chemically etched ormechanically punched to form a matrix 32 of heat spreaders 34interconnected one to another by heat spreader tie bars 36. The heatspreader 34 and heat spreader tie bars 36 have a typical thickness inthe range of 0.1 millimeter to 1.0 millimeter. The heat spreader 34 isformed from a ductile, high thermal conductivity metal such as copper,aluminum and alloys thereof. The heat spreader may be coated to impart acolor to enhance contrast for package marking or to impart enhanced andresistance to environmental corrosion. For example, when the heatspreader is formed from copper or a copper-base alloy, it may be coatedwith nickel by an electrolytic or electroless process. When formed fromaluminum or an aluminum alloy, it may be anodized, such as a blackanodization process. As shown in FIG. 3, subsequent to etching orpunching, portions 38 of the tie bars 36 may be mechanically formed toelevate the heat spreaders 34 relative to tie bars 36. This upsetelevates the heat spreader 34 for an amount effective to provideclearance from the wires of a wire bonded package and to enable top mostsurface 40 to be exposed following package molding if desired. A typicalamount of upset, u, is between 0.25 mm and 0.7 mm.

With reference to the insert expanding a portion of FIG. 2, portion 60of heat spreader tie bars 36 may imparted with a reduced thicknessduring etching to facilitate singulation. Such partial etching may alsobe used on that portion of lead tie bars that is cut during singulation.

With reference to FIG. 4, the array 32 of heat spreaders 34 is thenattached to a feature, such as leads 16 or tie bars to be in non-contactproximity to the die. The array 32 may be attached by an adhesive 42such as an epoxy or conductive tape. Adhesive 42 is optional and thearray 32 may be simply placed in position and held firm with a moldingresin.

FIG. 5 shows in cross-sectional representation an array 44 after amolding resin 46 has encapsulated the package. Encapsulated componentsand features include the die, at least a portion of the heat spreaderand all but an out lead end 47. A typical molding resin is a dielectricpolymer. The assembly of FIG. 4 is placed in a suitable mold and moldingresin at an elevated temperature is introduced into the mold forming thearray of packages 44 shown in FIG. 5. After encapsulation, the array ofpackages is singulated such as by sawing or punching to form individualpackages 48 as illustrated in FIG. 6.

The die 28 is disposed between two metallic plates, the die pad 14 andheat spreader 34. This provides shielding from both electric andmagnetic fields for electrically sensitive devices.

FIG. 7 illustrates a process flow to manufacture a wire bonded package70 in accordance with the invention. A lead frame 72 that may be amember of a matrix or a single lead frame is etched to possess desiredfeatures such as leads 74 and a die pad 76. To support the featuresfollowing etching, a backing strip 78, such as an adhesive tape isapplied.

An integrated circuit device 80 is bonded to an interior surface 82 ofdie pad 76 by a die attach 84. Typical die attach material includegold/tin alloy eutectics, gold/silver alloy eutectics, varioussilver-base alloys and metal filled polymers. Wire bonds 86 or TAB tapethen electrically interconnect leads 74 to I/O pads on an electricallyactive face of the integrated circuit device 80. The electrically activeface of the integrated circuit device 80 includes circuitry and I/O padswhile the opposing electrically inactive face is devoid of thesefeatures.

Heat spreader 88 is next positioned on the leads 74. Optionally, theheat spreader 88 is affixed to the leads 74 or lead frame tie bars by anadhesive 90 such as an epoxy or conductive tape. Such as an epoxy orconductive tape. A molding resin 91 then encapsulates the integratedcircuit device 80, at least a portion of the heat spreader 88 and aportion of the leads 74. At least one outer lead surface 92, 92′ isexposed and forms a planar surface with the sidewalls 94, 94′ of themolding resin. An outermost surface 96 of the heat spreader 88 may alsobe exposed and forms a planar surface with sidewall 94″ of the moldingresin.

If the lead frame and heat spreader were provided as members of amatrix, the final step is singulation. If single unit lead frame andheat spreader were used, then singulation is not required.

An enlarged view of the package 70 is illustrated in cross-sectionalrepresentation in FIG. 8. The package includes a thinned portion 60 ofthe heat spreader tie bars to facilitate singulation by sawing orpunching. A second thinned portion 96 mechanically locks the heatspreader 88 in molding resin 91.

A first alternative package 100 is illustrated in FIG. 9. In thispackage, the heat spreader 102 includes a plurality of apertures 104such that molding resin 91 projects through the apertures tomechanically lock the head spreader in the molding resin. The pluralityof apertures 104 may be used in combination with any of the packageconfigurations described herein.

A second alternative package 110 is illustrated in FIG. 10. In thispackage, the heat spreader 112 has a recessed central portion 114. Athermally conductive grease or adhesive such as an epoxy 116 orconductive tape provides good thermal conduction. The thermallyconductive grease or epoxy may be a dielectric or electricallyconductive depending on the application. When used as a wire bondreplacement, it is selected to be electrically conductive. If only forthermal dissipation and not intended to electrically interconnect to I/Opads, then it is selected to be a dielectric to prevent shorting.Peripheral portions 118 of the heat spreader form a planar surface witha sidewall 94″ of the package 110 to facilitate the removal of heat byforced air, thermal fluid or contact with an external heat sink.

A third alternative package 120 illustrated in FIG. 11 is similar to thepackage of FIG. 10 except that peripheral portions 118 of heat spreader112 do not form a portion of the sidewall 94″ of the package.

A fourth alternative package 130 is illustrated in FIG. 12. The package130 has a die pad 132 with a recessed central portion 134. Anelectrically conductive, thermally conductive adhesive such as a thermalgrease 136, epoxy, or conductive tape provides both electrical andthermal connectivity between an electrically active face of theintegrated circuit device 80 and heat spreader 138. An exemplaryelectrically conductive, thermally conductive thermal grease is anemulsion of ceramic or metal particles, such as silver, copper and/oraluminum based, in an organic or silicone fluid. Alternatively, thethermal grease 136 may be replaced with an electrically conductive,thermally conductive epoxy such as a silver filled epoxy or adispensable solder paste.

FIG. 13 illustrates a method for the assembly of a flip chip package 150in accordance with another embodiment of the invention. Most of theassembly steps for the package 150 are similar to the previouslydescribed steps. However, the electrically active face of the integratedcircuit device 80 is directly bonded to the leads 74, and optionally toa central die pad 182 (FIG. 18), by solder bumps 152. Referring back toFIG. 13, solder bumps 152 typically have the height of 0.07 mm and areformed from a suitable solder such as lead-base eutectic, high leadcontent and pillar bump. Projections 154, 154′ extend into the moldingresin 91 mechanically locking leads 74 and heat spreader 88 in place.

Alternative flip chip packages, 150, 160, 170, 180 embodiments of thepackages of the invention are shown in FIGS. 14 through 17. Most of thefeatures have been previously described. For the flip chip version, thethermal grease 136 is electrically and thermally conductive andelectrically and thermally interconnects the heat spreader 102, 112 andelectrically inactive face of integrated circuit device 80. As above,thermal epoxies, solder pastes, and conductive tape may substitute forthe thermal grease. One suitable thermal epoxy is filled with in excessof 60 weight percent of silver powder.

In both the flip chip version and the wire bonded/TAB bonded version, asurface 158 of the heat spreader of any of the heat spreaders 88 may beexposed to the environment forming a planar surface with a sidewall 94″surface of the molding resin 91. In addition to providing a markingsurface, the surface 158 may be exposed to forced air, a thermallyconductive fluid or a heat sink to improve thermal management. The shapeof the exposed surface may be square, rectangular, circular or any othershape.

Referring now to FIG. 19, heat spreader tie bars 190 may have bumps 192to enhance standoff clearance from the wires used for wire bonding.Bumps 192 are also useful to align and lock the heat spreader inposition on leads 194. Apertures 196 may be formed in the leads 194 tofurther enhance alignment and locking.

Alternatively, as shown in FIG. 20, bumps 192 may be formed in thepackage leads 194 or lead frame tie bars. Apertures 196 may be formed inheat spreader tie bars 190. The bumps 192 again function as alignmentand locking features. The bumps are typically formed during the chemicaletching process or by coining/punching during the upset process.

Referring now to FIG. 21, in another embodiment, package leads 194 mayinclude a bump 192 and heat spreader tie bar 190 may include an aperture196. Aperture 196 and bump 192 are configured to function as analignment and locking feature.

While the assembly process describes the array of leads and array ofheat spreaders being molded together and subsequently singulated, it iswithin the scope of the invention for the heat spreaders and leads to besingulated prior to encapsulation with the molding resin and a pick andplace process used to place individual lead frame assemblies andindividual heat spreaders in individual mold cavities for encapsulation.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the process may be used for the manufacture of a DFN package orto encapsulate one or more semiconductor devices and passive electricaldevices such as in a hybrid package. Accordingly, other embodiments arewithin the scope of the following claims.

1. A method for the manufacture of a no-lead electronic package,comprising: providing a lead frame having features including a pluralityof leads terminating about a central aperture; positioning an integratedcircuit device within said central aperture, the integrated circuitdevice having an electrically active face; electrically interconnectingsaid integrated circuit device to said leads; positioning a heatspreader corresponding to each said integrated circuit device innon-contact proximity to said electrically active face so that saidintegrated circuit device is disposed between said leads and said heatspreader; and encapsulating said semiconductor device, at least aportion of said heat spreader, and at least a portion of said leads in amolding resin, wherein the heat spreader has an outer portion in contactwith outer lead ends of said leads on opposite sides of the integratedcircuit device, the outer portion including a first thinned portion atan end thereof in contact with said outer lead ends, and a centralportion having a planar first surface opposite and substantiallyparallel to said electrically active face, the central portion includinga full thickness portion having a planar surface opposite the firstsurface and a second thinned portion spaced apart from the integratedcircuit device and having a lower surface integral with said firstsurface and an upper surface parallel to said first surface; saidencapsulating step includes encapsulating all of the second thinnedportion and leaving exposed part of the first thinned portion, andleaving exposed a portion of each of said outer lead ends and forming aplanar side surface with the exposed portion of an outer lead end, saidplanar side surface including said exposed part of the first thinnedportion; and the package has a bottom surface and a planar sidewallsubstantially perpendicular thereto, said sidewall including said planarside surface.
 2. A method according to claim 1 wherein said heatspreader is in a non-planar relationship with respect to a plurality oftie bars and said plurality of tie bars are supported by said packagefeatures.
 3. A method according to claim 2 wherein said heat spreader iscompletely encapsulated within said molding resin.
 4. A method accordingto claim 2 wherein a surface of said heat spreader is exposed and planarwith a surface of said molding resin.
 5. A method according to claim 4wherein said heat spreader is selected from the group consisting ofcopper, aluminum, copper-base alloys, and aluminum base alloys.
 6. Amethod according to claim 5 further comprising the step of coating saidheat spreader prior to encapsulation with molding resin.
 7. A methodaccording to claim 6 wherein said heat spreader is selected to be analuminum base alloy and said coating step is black anodization.
 8. Amethod according to claim 6 wherein said heat spreader is selected to bea copper-base alloy and said coating step includes applying a coating ofnickel.
 9. A method according to claim 2 further comprising the step ofbonding said tie bars to said features.
 10. A method according to claim9 wherein said features include a die pad disposed within said centralaperture and die pad tie bars extending outwardly from said die padwherein said heat spreader tie bars are adhesively bonded to at leastone of said leads and said die pad tie bars.
 11. A method according toclaim 10, wherein said heat spreader tie bars are adhesively bonded toat least one of said leads and said die pad tie bars using at least oneof an epoxy and a conductive tape.
 12. A method according to claim 10further comprising the step of bonding a non-electrically active face ofsaid integrated circuit device to said die pad and electricallyinterconnecting an electrically active face of said integrated circuitdevice to said leads by wire bonds or TAB bonds.
 13. A method accordingto claim 9 wherein said electrically active face of said semiconductordevice is directly bonded to said leads by flip chip bonding.
 14. Amethod according to claim 13 wherein a thermally conductive polymercontacts both said electrically inactive face of said integrated circuitdevice and said heat spreader.
 15. A method according to claim 9 whereinsaid heat spreader tie bars interconnect to adjoining heat spreader tiebars to form a heat spreader array.
 16. A method according to claim 15wherein said step of encapsulating in molding resin occurs aftersingulation of said heat spreader tie bars.
 17. A method according toclaim 15 wherein said step of encapsulating in molding resin occursbefore singulation of said heat spreader tie bars.
 18. A methodaccording to claim 15 wherein said heat spreader tie bars have a reducedthickness portion to facilitate singulation.
 19. A semiconductor packagecomprising: a plurality of leads having inner ends and outer endsdisposed about a centrally disposed aperture; an integrated circuitdevice spanning said aperture and having an electrically active facedirectly bonded to said inner ends of said plurality of leads by asolder; a heat spreader in non-contact proximity to an electricallyinactive face of said integrated circuit device, whereby said integratedcircuit device is disposed between said plurality of leads and said heatspreader; and a molding resin encapsulating said integrated circuitdevice, at least a portion of said heat spreader, and at least a portionof said outer ends, leaving exposed a portion of each of said outer leadends and forming a planar side surface with the exposed portion of anouter lead end, wherein the heat spreader has an outer portion incontact with the outer ends of leads on opposite sides of the die pad,the outer portion including a first thinned portion at an end thereof incontact with said outer lead ends, and a central portion having a planarfirst surface opposite and substantially parallel to said electricallyinactive face, the central portion including a full thickness portionhaving a planar surface opposite the first surface, and a second thinnedportion spaced apart from the die pad and having a lower surfaceintegral with said first surface and an upper surface parallel to saidfirst surface; said molding resin encapsulates all of the second thinnedportion and leaves exposed part of the first thinned portion, saidplanar side surface including said exposed part of the first thinnedportion; and the package has a bottom surface and a planar sidewallsubstantially perpendicular thereto, said sidewall including said planarside surface.
 20. A semiconductor package according to claim 19 whereinsaid heat spreader is selected from the group consisting of copper,aluminum, copper-base alloys, and aluminum-base alloys.
 21. Asemiconductor package according to claim 20 wherein said heat spreaderfurther comprises a coating, which is added to said heat spreader priorto encapsulation with said molding resin.
 22. A semiconductor packageaccording to claim 20 wherein said heat spreader is an aluminum-basealloy coated with a black anodization and a surface of said heatspreader is exposed in planar relationship with said molding resin. 23.A semiconductor package according to claim 20 wherein a thermallyconductive polymer is disposed between said electrically inactive faceand said heat spreader.
 24. A semiconductor package according to claim20 wherein a die pad is disposed within said aperture and saidintegrated circuit device is directly bonded to said die pad.
 25. Asemiconductor package according to claim 19 further comprising anadhesive for joining said heat spreader with at least a portion of saidplurality of leads.
 26. A semiconductor package according to claim 25,wherein said adhesive is one of an epoxy and a conductive tape.